1. Field of the Invention
The invention relates in general to a light emitting diode. More particularly, the invention relates to a light emitting diode comprising a meshed Ohmic contact layer to result in high light intensity.
2. Description of the Related Art
As the technique of light emitting diodes becomes more and more advanced, large area displays with high reliability have been fabricated. As the light emitting diodes used in large displays require very high light intensity, clarity is greatly demanded. In addition, the content of the display has to be observed from a long distance. To have a high light intensity and low power consumption is always the goal for developing light emitting diodes.
One of the major materials for fabricating light emitting diodes are direct bandgap materials such as aluminum gallium indium phosphide (AlGaInP). To match with the lattice of gallium arsenide (GaAs), an appropriate adjustment of the proportions of aluminum and gallium can adjust the wavelength of the luminescence to between 550 nm and 680 nm, that is, between green light and red light. As the addition of aluminum increases the bandgap, an aluminum gallium indium phosphide with high aluminum content is used as a confining layer to sandwich the central carrier luminescence layer or active layer. The carrier injection is thus enhanced, and the high efficiency double hetero-structure of a light emitting diode is formed. As the bandgap of the confining layer is larger than the energy of the generated photons, the light emitted from the active layer is not absorbed by the confining layer.
In FIG. 1, a typical aluminum gallium indium phosphide light emitting diode is shown. An n-type aluminum gallium indium phosphide confining layer 102, an aluminum gallium indium phosphide active layer 104, and a p-type aluminum gallium indium phosphide confining layer 106 are formed on a gallium arsenide substrate 100 using metal organic chemical vapor deposition (MOCVD). A front side electrode 108 and a rear side electrode 110 are then evaporated to complete the fabrication of the light emitting diode. To enhance the light intensity, a distributed Bragg reflector 112 is added under the n-type aluminum gallium indium phosphide confining layer 102. Thus constructed, the photons emitting to the n-type gallium arsenide substrate 100 are reflected to the front side to be output. As the p-type aluminum gallium indium phosphide confining layer 106 has the problem of low mobility and difficulty of doping, the resistivity is relatively high (about 0.5 Ohm-cm). Consequently, the lateral current cannot be effectively distributed around the whole chip. As the majority of carriers are injected right under the front side electrode 108, other positions of the active layer 104 cannot obtain enough carriers for radiative recombination which causes the luminescence. Furthermore, the current crowding effect also causes the majority of light to be blocked by the non-transparent front side electrode and reflected back to the bulk, or absorbed by the substrate 100 to cause a degradation of light efficiency.
In FIG. 2, to resolve the drawback of the light emitting diode as shown in FIG. 1, a current spreading layer 114 is added between the p-type aluminum gallium indium phosphide confining layer 106 and the front side electrode 108. In addition to the excellent transparency of the light emitting from the active layer 104, the current spreading layer 114 is easier doped than the p-type aluminum gallium indium phosphide confining layer 106 and has a higher mobility. As a result, the current can be distributed more evenly on the chip. Currently, aluminum gallium arsenide is used as the material for forming the current spreading layer 114. The thickness of the current spreading layer 114 is normally tens of microns in order to achieve the current spreading capacity. However, the metal organic chemical vapor deposition used to form such a current spreading layer 114 has a very slow growth rate. The fabrication cost is thus very high, and the fabrication time is long.
FIG. 3 illustrates structure to resolve the problems mentioned above and layer 114 is removed. A transparent conductive oxide layer 116 is formed as the current spreading layer of the light emitting diode. The transparent conductive oxide layer 116 not only has a great transparency, but also has an extremely low resistivity (about 3xc3x9710xe2x88x92xe2x88x9dOhm-cm). The current can thus be evenly distributed on the chip to enhance the light efficiency of the light emitting diode. The proper material of such transparent conductive oxide comprises indium oxide, tin oxide and indium tin oxide. To have a good Ohmic contact between the transparent conductive oxide layer and the semiconductor, and to increase adhesion, a heavily doped p-type Ohmic contact layer 118 is formed prior to formation of the transparent conductive oxide layer 116. The thickness of the Ohmic contact layer 118 is about 500 angstroms with a doping concentration larger than 10xe2x88x9218 cmxe2x88x923. The material for forming the Ohmic contact layer 118 includes gallium arsenide or gallium arsenic phosphide (GaAsP). Before forming the conductive oxide layer as the current spreading layer 116, it is necessary to grow the p-type Ohmic contact layer 118. However, as the energy bandgap of the Ohmic contact layer 118 is smaller than the energy bandgap of the active layer 104, the luminescence of the light emitting diode is absorbed by the Ohmic contact layer 118 to seriously affect the light intensity.
FIG. 4 shows the distribution of light absorption coefficients at different wavelengths for different materials. The light absorption coefficiency xcex1 of different compounds such as gallium indium arsenic phosphide (GaInAsP), gallium arsenide (GaAs), indium phosphide (InP), germanium (Ge), gallium phosphide (GaP) and silicon (Si) at 300 K can be seen.
The invention provides a light emitting diode. By a meshed structure, the area of light absorption of an Ohmic contact layer is greatly reduced. As a result, the light intensity of the light emitting diode is greatly enhanced.
The light emitting diode provided by the invention comprises a substrate with a first side and a second side. A distributed Bragg reflector, an n-type confining layer, an active layer, a p-type confining layer, a current spreading layer, a meshed Ohmic contact layer, a transparent conductive oxide layer and a front side electrode are formed on the first side of the substrate. A rear side electrode is formed on the second side of the substrate. As the Ohmic contact has an energy bandgap smaller than the luminescence of the active layer, the luminescence is thus absorbed thereby. However, by forming the Ohmic contact layer with a meshed structure, the area of absorbing the luminescence is effectively reduced. The light intensity of the lumicescence of the light emitting diode is thus enhanced.